The discussion of prior art information is herein divided into three parts in this Background section: (a) a discussion on high-capacity anode active materials for lithium-ion batteries and long-standing issues associated with these materials; (b) the new 2-D nano material called “graphene” and its prior use as a conductive additive material for the anode active material; and (c) graphene-based foamed material called “graphene foam”.
A unit cell or building block of a lithium-ion battery is typically composed of an anode active material layer, an anode or negative electrode layer (containing an anode active material responsible for storing lithium therein, a conductive additive, and a resin binder), an electrolyte and porous separator, a cathode or positive electrode layer (containing a cathode active material responsible for storing lithium therein, a conductive additive, and a resin binder), and a separate cathode current collector. The electrolyte is in ionic contact with both the anode active material and the cathode active material. A porous separator is not required if the electrolyte is a solid-state electrolyte.
The binder in the binder layer is used to bond the anode active material (e.g. graphite or Si particles) and a conductive filler (e.g. carbon black or carbon nanotube) together to form an anode layer of structural integrity, and to bond the anode layer to a separate anode current collector, which acts to collect electrons from the anode active material when the battery is discharged. In other words, in the negative electrode side of the battery, there are typically four different materials involved: an anode active material, a conductive additive, a resin binder (e.g. polyvinylidine fluoride, PVDF, or styrene-butadiene rubber, SBR), and an anode current collector (typically a sheet of Cu foil).
The most commonly used anode active materials for lithium-ion batteries are natural graphite and synthetic graphite (or artificial graphite) that can be intercalated with lithium and the resulting graphite intercalation compound (GIC) may be expressed as LixC6, where x is typically less than 1. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal corresponds to x=1, defining a theoretical specific capacity of 372 mAh/g.
Graphite or carbon anodes can have a long cycle life due to the presence of a protective solid-electrolyte interface layer (SEI), which results from the reaction between lithium and the electrolyte (or between lithium and the anode surface/edge atoms or functional groups) during the first several charge-discharge cycles. The lithium in this reaction comes from some of the lithium ions originally intended for the charge transfer purpose. As the SEI is formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e. these positive ions can no longer be shuttled back and forth between the anode and the cathode during charges/discharges. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer. In addition to SEI formation, the irreversible capacity loss Qir can also be attributed to graphite exfoliation caused by electrolyte/solvent co-intercalation and other side reactions.
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and the like, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions or react with lithium. Among these materials, lithium alloys having a composition formula of LiaA (A is a metal or semiconductor element, such as Al and Si, and “a” satisfies 0<a≤5) are of great interest due to their high theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, as schematically illustrated in FIG. 1(A), in an anode composed of these high-capacity materials, severe pulverization (fragmentation of the alloy particles) occurs during the charge and discharge cycles due to severe expansion and contraction of the anode active material particles induced by the insertion and extraction of the lithium ions in and out of these particles. The expansion and contraction, and the resulting pulverization, of active material particles, lead to loss of contacts between active material particles and conductive additives and loss of contacts between the anode active material and its current collector. This degradation phenomenon is illustrated in FIG. 1(A). These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, three technical approaches have been proposed:                (1) reducing the size of the active material particle, presumably for the purpose of reducing the total strain energy that can be stored in a particle, which is a driving force for crack formation in the particle. However, a reduced particle size implies a higher surface area available for potentially reacting with the liquid electrolyte to form a higher amount of SEI. Such a reaction is undesirable since it is a source of irreversible capacity loss.        (2) depositing the electrode active material in a thin film form directly onto a current collector, such as a copper foil. However, such a thin film structure with an extremely small thickness-direction dimension (typically much smaller than 500 nm, often necessarily thinner than 100 nm) implies that only a small amount of active material can be incorporated in an electrode (given the same electrode or current collector surface area), providing a low total lithium storage capacity and low lithium storage capacity per unit electrode surface area (even though the capacity per unit mass can be large). Such a thin film must have a thickness less than 100 nm to be more resistant to cycling-induced cracking, further diminishing the total lithium storage capacity and the lithium storage capacity per unit electrode surface area. Such a thin-film battery has very limited scope of application. A desirable and typical electrode thickness is from 100 μm to 200 μm. These thin-film electrodes (with a thickness of <500 nm or even <100 nm) fall short of the required thickness by three (3) orders of magnitude, not just by a factor of 3.        (3) using a composite composed of small electrode active particles protected by (dispersed in or encapsulated by) a less active or non-active matrix, e.g., carbon-coated Si particles, sol gel graphite-protected Si, metal oxide-coated Si or Sn, and monomer-coated Sn nano particles. Presumably, the protective matrix provides a cushioning effect for particle expansion or shrinkage, and prevents the electrolyte from contacting and reacting with the electrode active material. Examples of anode active particles are Si, Sn, and SnO2. Unfortunately, when an active material particle, such as Si particle, expands (e.g. up to a volume expansion >300%) during the battery charge step, the protective coating is easily broken due to the mechanical weakness and/o brittleness of the protective coating materials. There has been no high-strength and high-toughness material available that is itself also lithium ion conductive.        
It may be further noted that the coating or matrix materials used to protect active particles (such as Si and Sn) are carbon, sol gel graphite, metal oxide, monomer, ceramic, and lithium oxide. These protective materials are all very brittle, weak (of low strength), and/or non-conducting (e.g., ceramic or oxide coating). Ideally, the protective material should meet the following requirements: (a) The coating or matrix material should be of high strength and stiffness so that it can help to refrain the electrode active material particles, when lithiated, from expanding to an excessive extent. (b) The protective material should also have high fracture toughness or high resistance to crack formation to avoid disintegration during repeated cycling. (c) The protective material must be inert (inactive) with respect to the electrolyte, but be a good lithium ion conductor. (d) The protective material must not provide any significant amount of defect sites that irreversibly trap lithium ions. (e) The protective material must be lithium ion-conducting as well as electron-conducting. The prior art protective materials all fall short of these requirements. Hence, it was not surprising to observe that the resulting anode typically shows a reversible specific capacity much lower than expected. In many cases, the first-cycle efficiency is extremely low (mostly lower than 80% and some even lower than 60%). Furthermore, in most cases, the electrode was not capable of operating for a large number of cycles. Additionally, most of these electrodes are not high-rate capable, exhibiting unacceptably low capacity at a high discharge rate. Due to these and other reasons, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction steps, and other undesirable side effects.
Complex composite particles of particular interest are a mixture of separate Si and graphite particles dispersed in a carbon matrix; e.g. those prepared by Mao, et al. [“Carbon-coated Silicon Particle Powder as the Anode Material for Lithium Batteries and the Method of Making the Same,” US 2005/0136330 (Jun. 23, 2005)]. Also of interest are carbon matrix-containing complex nano Si (protected by oxide) and graphite particles dispersed therein, and carbon-coated Si particles distributed on a surface of graphite particles Again, these complex composite particles led to a low specific capacity or for up to a small number of cycles only. It appears that carbon by itself is relatively weak and brittle and the presence of micron-sized graphite particles does not improve the mechanical integrity of carbon since graphite particles are themselves relatively weak. Graphite was used in these cases presumably for the purpose of improving the electrical conductivity of the anode material. Furthermore, polymeric carbon, amorphous carbon, or pre-graphitic carbon may have too many lithium-trapping sites that irreversibly capture lithium during the first few cycles, resulting in excessive irreversibility.
In summary, the prior art has not demonstrated a composite material that has all or most of the properties desired for use as an anode material in a lithium-ion battery. Thus, there is an urgent and continuing need for a new anode for the lithium-ion battery that has a high cycle life, high reversible capacity, low irreversible capacity, small particle sizes (for high-rate capacity), and compatibility with commonly used electrolytes. There is also a need for a method of readily or easily producing such an anode in a cost-effective manner.
Bulk natural graphite is a 3-D graphitic material with each graphite particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). Although all the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are inclined at different orientations. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
The constituent graphene planes of a graphite crystallite in a natural or artificial graphite particle can be exfoliated and extracted or isolated to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome. An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of approximately 0.3354 nm is commonly referred to as a multi-layer graphene. A multi-layer graphene platelet has up to 300 layers of graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Graphene or graphene oxide sheets/platelets (collectively, NGPs) are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
In one process, graphene materials are obtained by intercalating natural graphite particles with a strong acid and/or an oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 2 (a process flow chart). The presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing (d002, as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction. The GIC or GO is most often produced by immersing natural graphite powder (20 in FIG. 2) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate). The resulting GIC (22) is actually some type of graphite oxide (GO) particles if an oxidizing agent is present during the intercalation procedure. This GIC or GO is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. In order to produce graphene materials, one can follow one of the two processing routes after this rinsing step, briefly described below:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by a factor of 30-300 to form “graphite worms” (24), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected.
In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets or foils (26) that typically have a thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm). Alternatively, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes” (49) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition).
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 10 nm (commonly referred to as few-layer graphene). Multiple graphene sheets or platelets may be made into a sheet of NGP paper (34) using a paper-making process.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.001%-10% by weight, more typically 0.01%-5% by weight, most typically and preferably less than 2% by weight.
For the purpose of defining the claims of the instant application, NGPs or graphene materials include discrete sheets/platelets of single-layer and multi-layer (typically less than 10 layers) pristine graphene, graphene oxide, reduced graphene oxide (RGO), graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, doped graphene (e.g. doped by B or N). Pristine graphene has essentially 0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight. Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen. Other than pristine graphene, all the graphene materials have 0.001%-50% by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.). These materials are herein referred to as non-pristine graphene materials.
Another process for producing graphene, in a thin film form (typically <2 nm in thickness), is the catalytic chemical vapor deposition process. This catalytic CVD involves catalytic decomposition of hydrocarbon gas (e.g. C2H4) on Ni or Cu surface to form single-layer or few-layer graphene. With Ni or Cu being the catalyst, carbon atoms obtained via decomposition of hydrocarbon gas molecules at a temperature of 800-1,000° C. are directly deposited onto Cu foil surface or precipitated out to the surface of a Ni foil from a Ni—C solid solution state to form a sheet of single-layer or few-layer graphene (less than 5 layers). The Ni- or Cu-catalyzed CVD process does not lend itself to the deposition of more than 5 graphene planes (typically <2 nm) beyond which the underlying Ni or Cu layer can no longer provide any catalytic effect. The CVD graphene films are extremely expensive.
Our research group also pioneered the application of graphene materials for battery applications: One of our earlier applications discloses a graphene-based composite composition for use as a lithium ion battery anode [A. Zhamu and B. Z. Jang, “Nano Graphene Platelet-Based Composite Anode Compositions for Lithium Ion Batteries,” U.S. patent application Ser. No. 11/982,672 (Nov. 5, 2007); Now U.S. Pat. No. 7,745,047 (Jun. 29, 2010)]. This composition comprises: (a) micron- or nanometer-scaled particles or coating of an anode active material; and (b) a plurality of nano-scaled graphene platelets (NGPs), wherein a platelet comprises a graphene sheet or a stack of graphene sheets having a platelet thickness less than 100 nm and wherein the particles or coating are physically attached or chemically bonded to NGPs. Nano graphene platelets (NGPs) are individual graphene sheets (individual basal planes of carbon atoms isolated from a graphite crystal) or stacks of multiple graphene planes bonded together in the thickness direction. The NGPs have a thickness from 0.34 nm to 100 nm and a length, width, or diameter that can be greater or less than 10 μm. The thickness is more preferably less than 10 nm and most preferably less than 1 nm.
Disclosed in another patent application of ours is a more specific composition, which is composed of a 3-D network of NGPs and/or other conductive filaments and anode active material particles that are bonded to these NGPs or filaments through a conductive binder [J. Shi, A. Zhamu and B. Z. Jang, “Conductive Nanocomposite-based Electrodes for Lithium Batteries,” U.S. patent application Ser. No. 12/156,644 (Jun. 4, 2008)]. Yet another application provides a nano graphene-reinforced nanocomposite solid particle composition containing NGPs and electrode active material particles, which are both dispersed in a protective matrix (e.g. a carbon matrix) [A. Zhamu, B. Z. Jang, and J. Shi, “Nano Graphene Reinforced Nanocomposite for Lithium Battery Electrodes,” U.S. patent application Ser. No. 12/315,555 (Dec. 4, 2008)].
After our discovery of graphene providing an outstanding support for anode active materials, many subsequent studies by others have confirmed the effectiveness of this approach. For instance, Wang, et al. investigated self-assembled TiO2-graphene hybrid nanostructures for enhanced Li-ion insertion [D. Wang, et al. “Self-Assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion.” ACS Nano, 3 (2009) 907-914]. The results indicate that, as compared with the pure TiO2 phase, the specific capacity of the hybrid was more than doubled at high charge rates. The improved capacity at a high charge-discharge rate was attributed to increased electrode conductivity afforded by a percolated graphene network embedded into the metal oxide electrodes. However, all these earlier studies were focused solely on providing a network of electron-conducting paths for the anode active material particles and failed to address other critical issues, such as ease of anode material processing, electrode processability, electrode tap density (the ability to pack a dense mass into a given volume), stability of solid-electrolyte interface (SEI), and long-term cycling stability. For instance, the method of preparing self-assembled hybrid nanostructures is not amenable to mass production. These are all critically important issues that must be addressed in a real battery manufacturing environment.
The present invention goes beyond and above these prior art efforts of using solid graphene sheets or platelets (NGPs) to form a 3-D conductive network to support an anode active material. Specifically, the instant application makes use of a graphene foam material to protect an anode active material (i.e., Si nanowires) by providing several other unexpected functions, in addition to forming a 3-D network of electron-conducting pathways. Hence, a brief discussion on the production of graphene foams should be helpful to the reader.
Generally speaking, a foam (or foamed material) is composed of pores (or cells) and pore walls (the solid portion of a foam material). The pores can be interconnected to form an open-cell foam. A graphene foam is composed of pores and pore walls that contain a graphene material. There are three major methods of producing graphene foams:
The first method is the hydrothermal reduction of graphene oxide hydrogel that typically involves sealing graphene oxide (GO) aqueous suspension in a high-pressure autoclave and heating the GO suspension under a high pressure (tens or hundreds of atm) at a temperature typically in the range of 180-300° C. for an extended period of time (typically 12-36 hours). A useful reference for this method is given here: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are several major issues associated with this method: (a) The high pressure requirement makes it an impractical method for industrial-scale production. For one thing, this process cannot be conducted on a continuous basis. (b) It is difficult, if not impossible, to exercise control over the pore size and the porosity level of the resulting porous structure. (c) There is no flexibility in terms of varying the shape and size of the resulting reduced graphene oxide (RGO) material (e.g. it cannot be made into a film shape). (d) The method involves the use of an ultra-low concentration of GO suspended in water (e.g. 2 mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to 50%), one can only produce less than 2 kg of graphene material (RGO) per 1000-liter suspension. Furthermore, it is practically impossible to operate a 1000-liter reactor that has to withstand the conditions of a high temperature and a high pressure. Clearly, this is not a scalable process for mass production of porous graphene structures.
The second method is based on a template-assisted catalytic CVD process, which involves CVD deposition of graphene on a sacrificial template (e.g. Ni foam). The graphene material conforms to the shape and dimensions of the Ni foam structure. The Ni foam is then etched away using an etching agent, leaving behind a monolith of graphene skeleton that is essentially an open-cell foam. A useful reference for this method is given here: Zongping Chen, et al., “Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition,” Nature Materials, 10 (June 2011) 424-428. There are several problems associated with such a process: (a) the catalytic CVD is intrinsically a very slow, highly energy-intensive, and expensive process; (b) the etching agent is typically a highly undesirable chemical and the resulting Ni-containing etching solution is a source of pollution. It is very difficult and expensive to recover or recycle the dissolved Ni metal from the etchant solution. (c) It is challenging to maintain the shape and dimensions of the graphene foam without damaging the cell walls when the Ni foam is being etched away. The resulting graphene foam is typically very brittle and fragile. (d) The transport of the CVD precursor gas (e.g. hydrocarbon) into the interior of a metal foam can be difficult, resulting in a non-uniform structure, since certain spots inside the sacrificial metal foam may not be accessible to the CVD precursor gas. ( ) This method does not lend itself to embedding anode active material particles therein.
The third method of producing graphene foam also makes use of a sacrificial material (e.g. colloidal polystyrene particles, PS) that is coated with graphene oxide sheets using a self-assembly approach. For instance, Choi, et al. prepared chemically modified graphene (CMG) paper in two steps: fabrication of free-standing PS/CMG films by vacuum filtration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μm PS spheres), followed by removal of PS beads to generate 3D macro-pores. [B. G. Choi, et al., “3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities,” ACS Nano, 6 (2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standing PS/CMG paper by filtration, which began with separately preparing a negatively charged CMG colloidal and a positively charged PS suspension. A mixture of CMG colloidal and PS suspension was dispersed in solution under controlled pH (=2), where the two compounds had the same surface charges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV for PS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) and PS spheres (zeta potential=+51±2.5 mV) were assembled due to the electrostatic interactions and hydrophobic characteristics between them, and these were subsequently integrated into PS/CMG composite paper through a filtering process. This method also has several shortcomings: (a) This method requires very tedious chemical treatments of both graphene oxide and PS particles. (b) The removal of PS by toluene also leads to weakened macro-porous structures. (c) Toluene is a highly regulated chemical and must be treated with extreme caution. (d) The pore sizes are typically excessively big (e.g. several μm), too big for many useful applications.
The above discussion clearly indicates that every prior art method or process for producing graphene foams has major deficiencies. Further, none of the earlier work makes use of graphene foam as a protective material for an anode active material of a lithium battery.
Thus, it is an object of the present invention to provide a cost-effective process for producing highly conductive, mechanically robust graphene foams in large quantities. This graphene foam also contains an anode active material (Si nanowires) residing in the pores of this foam and being protected by this foam. Some of these Si nanowires are chemically bonded to graphene planes that constitute foam walls. This process does not involve the use of an environmentally unfriendly chemical. This process enables flexible design and control of the porosity level and pore sizes, alleviating the volume expansion issues of the anode layer caused by Si expansion commonly associated with the high-capacity anode of a lithium-ion battery.
It is another object of the present invention to provide a process for producing graphene foam-protected Si nanowires wherein the graphene foam exhibits a thermal conductivity, electrical conductivity, elastic modulus, and/or compressive strength that is comparable to or greater than those of the graphite/carbon foams. The internal pores of the protective graphene foam expands and shrinks congruently with the expansion and shrinkage of the embedded Si nanowires, enabling long-term cycling stability of a lithium battery featuring such a high-capacity anode.
It is another object of the present invention to provide an anode electrode layer that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, and elasticity unmatched by any anode layer commonly used in a lithium-ion battery. There is no binder (e.g. SBR or PVDF) or conductive additive (e.g. acetylene black) that are required or included in the anode electrode. The high conductivity enables the use of the graphene foam host itself as an anode current collector, obviating the need to have a separate (additional) current collector (e.g. Cu foil) and thus reducing the weight and volume of the battery. Furthermore, there is no limitation on the achievable thickness of the foam layer (e.g, greater than 500 μm or several mm), in contrast to the thickness of typically 70-200 μm for conventional lithium-ion battery anodes. These features enable the production of lithium-ion batteries having significantly higher gravimetric and volumetric energy densities.